Metamorphosis allows one organism to fit two different niches. In the case of caterpillar vs butterfly, the organism gets all the benefits of a low energy leaf-eater (fast-growing, durable, ready food supply, not much wasted energy) AND all the benefits of a high-energy flying organism (different food supply, ability to lay eggs in protected places far, far away.) This is a big survival advantage, so the trait is retained.

It fits Darwin's theory because we can outline a gradual step by step process by which it could evolve, based on the fossil record and existing species, which retain some of the same traits in a more primitive form.

There is a wealth of scientific information out there on the internet. Sixty seconds worth of searching revealed these relevant scientific[sup]*[/sup] articles. Further searching will reveal even more. (Some are not free, but if you’re serious about learning you can either (i) pay for the online version, or (ii) go to a library and photocopy the articles.)

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[sup]*[/sup] I stress “scientific” sources because, as with most evolutionary topics, idiot creationists have bastardized and corrupted the topic to fit their pre-determined non-scientistic viewpoint. Be wary of where you get your information from.

Metamorphosis allows one organism to fit two different niches. In the case of caterpillar vs butterfly, the organism gets all the benefits of a low energy leaf-eater (fast-growing, durable, ready food supply, not much wasted energy) AND all the benefits of a high-energy flying organism (different food supply, ability to lay eggs in protected places far, far away.) This is a big survival advantage, so the trait is retained.

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Well I just took as if caterpillar and a butterfly are like different specie , are they not? we know they look different ?

So do babies and adults, but they are not different species.
Caterpillars are the laval form of the butterfly.

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Call them what ever you want , they are different ,babe is human and an adult is human babe walk adult don't fly . Caterpillar don't fly butterfly.
You can argue any way you want they are different, The only thing I want ask is how can you explain evolution in this case , and not using a fancy word metamorphosis.

Most critters have way more genes then they use. The caterpillar begins its life using one part of its entire genes. The metamorphosis shuts off some genes and turns on others. The net result is it uses more genes over its life.

Evolution already implies the same answer for everything. Science that is conducted to support evolution, will make use of this template even before the investigation begins. This is why they all sound the same. The butterfly does not fit the template in quite the same way.

Caterpillars can defoliate large areas. Their metamorphosis into a butterfly will not lower their stress on an ecosystem, but also allows the caterpillars of the future to move their eating show to another fresh location in record time.

The caterpillars, if they did not change, might strip an area of leaves, leading to their own demise. They will no longer be as hidden from birds and could run out of food. By becoming a butterfly, they are able to relocate far enough away to get fresh food, while allowing the stripped area to regrow. As the butterfly they also change their diet into nectar and help pollinate to make up for their destruction.

It fits Darwin's theory because we can outline a gradual step by step process by which it could evolve, based on the fossil record and existing species, which retain some of the same traits in a more primitive form.

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Sure.

First step - single life stage without shedding. (i.e. ants.) They are hatched with the same basic form, then grow as soft, weak insects, then their exoskeleton hardens and they cannot grow any longer.

Second - single life stage with shedding (i.e. some beetles.) They are hatched small, and a hard shell forms almost immediately. When they get too big for their skin they shed it and get larger. This has evolutionary advantages - they can form hard protective shells right away, are stronger right away, and can still grow to be bigger and stronger.

Third - multiple life stages with shedding. (Dragonflies.) These are hatched small and immediately form a hard shell for protection. They then moult once, and during that moult, get larger _and_ slightly change their form as new features grow. Dragonflies grow wings, for example. This is a big advantage since the egg doesn't have to contain enough energy to grow the wings at first; the nymph can eat to get enough energy to moult.

Fourth - metamorphosis. As the organism evolves, the change from nymph to adult becomes more and more drastic, and takes longer and longer. Nymphs that just stop moving while that long final moult happens get eaten, so there is an evolutionary advantage to protecting yourself during that time (digging a burrow, making a cocoon.)